Mercury is an undesirable constituent of all natural gas streams, and consequently a considerable number of methods have been devised to selectively remove the mercury. In the main the mercury impurity is in the form of elemental mercury, but in at least some instances mercury compounds, including organic mercury compounds, are also present. In the case of elemental mercury the purification processes are largely adsorption procedures, and in these the most common type of adsorbent is an activated carbon having supported thereon a mercury reactive material such as potassium triodide, sulfur, sulfuric acid, chlorine, silver, copper or various salts of silver or copper. Other supports for the mercury reactive materials include silicas, aluminas, silica-aluminas and zeolitic aluminosilicates. Ion-exchange resins, particularly the strongly basic anion-exchange types which have been reacted with a polysulfide, have also been reported as useful mercury adsorbents. See U.S. Pat. No. 4,591,490 (Horton) in this latter regard. The disclosures of U.S. Pat. No. 4,500,327 (Nishino) and U.S. Pat. No. 4,196,173 (de Jong et al) are pertinent to the use of activated carbon supports.
Perhaps the two greatest problems involved in removing mercury from natural gas streams are (a) achieving a sufficient reduction in the mercury concentration of the feed stream being treated, and (b) avoiding the reentry of the recovered mercury into some other environmental medium. Although permissible levels of mercury impurity vary considerably, depending upon the ultimate intended use of the purified product, for purified natural gas, a mercury concentration greater than about 0.01 microgram per normal cubic meter (.mu.g/nm.sup.3) is considered undesirable, particularly in those instances in which the natural gas is to be liquefied by cryogenic processing. To attain low concentration levels requires the use of relatively large adsorption beds and relatively low mercury loadings. If non-regenerable, the capital and adsorbent costs are uneconomical, and if regenerable, the regeneration media requirements are not only large but result in a large mercury-laden bed effluent which must itself be disposed of in an environmentally safe manner.